Researcher uses bullets to study plastics

HOUSTON (Nov. 20, 11:45 a.m. ET) — Researchers at Rice University and the Massachusetts Institute of Technology have carried out nanoscale investigations into what happens when bullets hit objects in an attempt to learn more about material properties.

The research, led by Ned Thomas, dean of the George Brown School of Engineering at Rice, uncovered a “surprising amount of information” about how block copolymers, such as polyurethane, dissipate the strain of sudden impact.

The goal is to find novel ways to make materials more impervious to deformation or failure, which can help make stronger and lighter body armor, jet engine turbine blades for aircraft, and cladding to protect spacecraft and satellites from micrometeorites and space junk.

The researchers were inspired by their observations in macroscopic ballistic tests in which a complex multiblock copolymer polyurethane material showed the ability to not only stop a 9 mm bullet but also seal the entryway behind it.

“The polymer has actually arrested the bullet and sealed it. There’s no macroscopic damage; the material hasn’t failed; it hasn’t cracked. You can still see through it. This would be a great ballistic windshield material,” Thomas said in a news release.

“We want to find out why this polyurethane works the way it does. Theoretically, no one understood why this particular kind of material – which has nanoscale features of glassy and rubbery domains – would be so good at dissipating energy,” he said.

One problem, Thomas said, is that cutting the polymer to analyze it on the nanoscale “would take days.” The researchers sought a model material that would react similarly at the nanoscale and could be analyzed much faster. They found one in a polystyrene-polydimethylsiloxane diblock-copolymer, which self-assembles into alternating 20-nanometer layers of glassy and rubbery polymers, and the disruption pattern from impact can be clearly seen.

The results showed several expected deformation mechanisms and the unexpected result that for sufficiently high velocities, the layered material melted into a homogeneous liquid that seemed to help arrest the projectile and, like the polymer, seal its entry path. The copolymer also behaved differently depending on where the spheres hit. The material showed the best ability to dissipate the energy of impact when spheres were fired perpendicular to the layers, Thomas said.

Thomas would like to extend testing to other lightweight, nanostructured materials like boron nitride, carbon nanotube-reinforced composites and graphite and graphene-based materials. The ultimate goal, he said, is to accelerate the design of metamaterials with precise control of their nano- and microstructures for a variety of applications.